Chemical vapor deposition of high conductivity, adherent thin films of ruthenium

ABSTRACT

A multi-step method for depositing ruthenium thin films having high conductivity and superior adherence to the substrate is described. The method includes the deposition of a ruthenium nucleation layer followed by the deposition of a highly conductive ruthenium upper layer. Both layers are deposited using chemical vapor deposition (CVD) employing low deposition rates.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 USC 120 of U.S. patentapplication Ser. No. 10/803,750 filed Mar. 18, 2004 now U.S. Pat. No.7,285,308, which in turn claims the benefit of priority under 35 USC 119of U.S. Provisional Patent Application No. 60/546,801 for “ChemicalVapor Deposition of High Conductivity, Adherent Thin Films of Ruthenium”filed on Feb. 23, 2004 in the name of Bryan C. Hendrix et al. Thedisclosures of said applications are hereby incorporated herein byreference, in their respective entireties, for all purposes.

FIELD OF THE INVENTION

The present invention relates to the chemical vapor deposition ofruthenium thin films. More specifically, the present invention relatesto a multi-step process for the chemical vapor deposition of rutheniumthin films, including the deposition of a ruthenium nucleation layerthat provides superior adhesion to, and continuous coverage of, thesubstrate followed by the deposition of a highly conductive rutheniumupper layer that is adherent to the nucleation layer.

DESCRIPTION OF THE RELATED ART

The noble metal ruthenium (Ru) is being widely investigated for use inconductive layers of integrated circuits (IC). In particular, layerscomprising ruthenium are being investigated for use as the lower(bottom) electrode of IC capacitors that may be used, for example, inDynamic Random Access Memories (DRAM). In addition, ruthenium thin filmsare used as the high work function gate electrode material for dualmetal gates and as a direct-platable barrier material for Cu-low k dualdamascene interconnects.

The major challenges in depositing ruthenium films include obtaining aruthenium thin film that is highly conductive, i.e., having lowresistance, while displaying good adhesion to the substrate, e.g.,dielectric, surface and high conformality. In particular, the depositionof very thin ruthenium films (<10 nm) that are smooth and continuous hasbeen challenging.

Traditionally, sputtering methods have been used to deposit rutheniumfilms, yielding layers having good surface morphology and lowresistance. However, sputtering is limited by poor step coverage whenthe critical dimensions (CD) are small and the aspect ratios are high,which may be disadvantageous when forming three-dimensional highcapacitance electrode structures such as cylinder-type or fin-shapedcapacitor electrodes or when producing direct-platable barriers on highaspect ratio structures, e.g., copper interconnects. Additionally,although sputtered ruthenium has been shown to possess the necessarywork function for p-MOS transistors, the potential for ion damage to thetransistor channel precludes the consideration of sputtering for massproduction.

Chemical vapor deposition (CVD) is also used to form ruthenium layers.In CVD, ruthenium is deposited on an IC substrate, e.g., a dielectriclayer, using a gasified ruthenium source and a co-reactant gas. CVD ofthin ruthenium films achieves good step coverage and wafer-to-waferrepeatability on complicated topography. Unfortunately, a rutheniumlayer formed by CVD may have poor surface morphology or high impuritycontent and as such, a high electrical resistivity. If the rutheniumlayer is the bottom electrode of a MIM capacitor, poor surfacemorphology can result in high leakage current in the capacitor.

To increase the adherence of a ruthenium film to the substrate surface,sticking layers and seed layers have been proposed. For example, whenfabricating MIM capacitor stacks, a titanium nitride (TiN) stickinglayer may be deposited onto a substrate followed by deposition of theruthenium electrode on the sticking layer. However, the sticking layerproduced is often too thick for increasingly fine feature sizes, and assuch has limited applicability, or alternatively the sticking layermodifies the effective work function of the metal.

U.S. Pat. No. 6,479,100 to Jin et al. discloses a method of depositing aruthenium seed layer on a substrate using CVD. According to Jin et al.,the deposited ruthenium seed layers should contain oxygen to improveadherence to the substrate, but the characteristics of the subsequentlydeposited thin ruthenium film onto the seed layer were enhanced when theoxygen content of the seed layer is relatively low. Towards that end,Jin et al. disclosed that the deposited ruthenium oxide seed layer mustbe annealed prior to deposition of the pure ruthenium thin metal filmthereon. However, the reported resistivity of the deposited thin filmswas 50 to 55 μΩ-cm for 130 to 140 Å films, which is a factor of sevenworse than the resistivity for bulk ruthenium (7.4 μΩ-cm) and is worsethan the resistivity for bulk RuO₂ (44 μΩ-cm).

Other prior art methods teach the two-step deposition of ruthenium thinfilms, wherein the rate of deposition is high to ensure completecoverage of the substrate surface. However, it is well known in thechemical arts that rapidly grown crystals and crystal layers tend toincorporate impurities and other point defects leading to inferiorelectrical properties relative to those grown slowly.

It would therefore be a significant advance in the art to provide achemical vapor deposition method for depositing ruthenium thin filmswherein the deposited ruthenium films are thin, highly conformal, highlyconductive, and highly adhesive to all underlying materials, whileminimizing the cost of ownership of the process.

SUMMARY OF THE INVENTION

The present invention relates to the chemical vapor deposition ofruthenium thin films. More specifically, the present invention relatesto a multi-step process for the chemical vapor deposition of rutheniumthin films, including the deposition of a ruthenium nucleation layerthat provides superior adhesion to, and continuous coverage of, thesubstrate followed by the deposition of a highly conductive rutheniumupper layer that is adherent to the nucleation layer.

In one aspect, the invention relates to a method for depositing aruthenium thin film onto a substrate, said method comprising:

-   -   (a) depositing a nucleation layer comprising ruthenium onto the        substrate by chemical vapor deposition, wherein the nucleation        layer is deposited using a nucleation layer gas mixture under        nucleation layer CVD conditions; and    -   (b) depositing an upper layer comprising ruthenium onto the        nucleation layer by chemical vapor deposition, wherein the upper        layer is deposited using an upper layer gas mixture under upper        layer CVD conditions.

In another aspect, the invention relates to a method for depositing aruthenium thin film onto a substrate, said method comprising:

-   -   (a) depositing a nucleation layer comprising ruthenium onto the        substrate by chemical vapor deposition, wherein the nucleation        layer is deposited using a nucleation layer gas mixture in an        oxidizing environment under nucleation layer CVD conditions; and    -   (b) deoxygenating the nucleation layer comprising ruthenium in a        reducing environment,    -   wherein (a) and (b) are repeated sequentially and continuously        until the ruthenium thin film of desired thickness is deposited        onto the substrate.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the process conditions as a function of time(arbitrary units) associated with deposition process 7.

FIG. 2 illustrates the resistivity of films as a function of filmthickness for ruthenium thin films deposited according to CVD process 7described herein.

FIG. 3 is a schematic of process variables as a function of time(arbitrary units) for the pulsed process described in Example 5.

FIG. 4 is a plot of the resistivity of the films as a function of filmthickness. (♦) continuous first deposition step at 280° C., (·)continuous first deposition step at 300° C., (●) ruthenium thin filmdeposited in Example 3, (▴) pulsed process of Example 5 at 280° C., (▪)pulsed process of Example 5 at 300° C.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

The present invention relates to multi-step methods for depositingruthenium thin films having high conductivity and superior adherence tothe substrate. The method includes the deposition of a rutheniumnucleation layer under conditions that provide superior adhesion to, andcontinuous coverage of, the substrate surface followed by the depositionof a highly conductive ruthenium upper layer that is adherent to thenucleation layer. Both layers are deposited using chemical vapordeposition (CVD).

As defined herein, “peel-resistant layer films” are films deposited onsubstrates according to the methods taught herein that arenon-delaminated by application of tape and peel removal thereof.

In general, when a ruthenium film is formed by CVD, the surfacemorphology of the deposited film may vary depending on the processconditions, including substrate temperature, chamber pressure, precursorflow rate and reactive gas flow rates. Importantly, the morphology ofthe deposited film influences the electrical characteristics, e.g., thesheet resistance of the film.

It is well known in the art that smooth, continuous films are preferredas barrier layers or electrical contacts. When the deposition rate onthe substrate surface is less than the deposition rate on nuclei presenton the substrate surface, i.e., the rate of growth is greater in adirection perpendicular to the film, poor coverage of the substrate maybe observed even though the film may be quite thick. Under these growthconditions, what is observed are various grains, or islands, on thesubstrate surface, that may or may not be interconnected. In this case,the sheet resistance of the film is controlled by the trickle of currentthrough the various contacts between grains rather than the resistivityof the grains themselves. As such, it is preferable that as a film growson a substrate, the grains grow laterally, covering the entire substratesurface at a rate as great, or greater than, the perpendicular growth ofthe film at nuclei. This ensures that the grains are substantiallyphysically, and thus electrically, interconnected so that the sheetresistance is dictated by the resistivity of the grains themselves.

When a ruthenium film is used as an electrode in a three-dimensionalcapacitor electrode in a semiconductor device having a high integrationdensity, a rough film morphology affects the leakage and voltagebreakdown properties of the capacitor. It is generally desirable todeposit a uniform thin film having a thickness of less than 1000 Å, withgood step coverage and excellent electrical characteristics. In view ofthe foregoing discussion, it may be difficult to use a ruthenium filmconsisting essentially of poorly contacting grains as a capacitorelectrode.

In addition to film morphology considerations, ruthenium films formed byCVD typically include small amounts of carbon (from the rutheniumsource) and oxygen (from the co-reactant gas) as impurities. Impuritiesmay deteriorate the electrical characteristics of the depositedruthenium film by increasing the resistance of the ruthenium film.Therefore, it is preferable that the amount of impurities contained inthe deposited ruthenium film is kept low.

It is well-known in the art that full coverage of the substrate isenhanced by high oxygen levels in the process gas, surface reactionrate-limited depositions and low temperatures (which enhances thenucleation rate). However, regardless of the coverage enhancer chosen,high residual impurity films are deposited. For example, when depositingruthenium in the presence of high oxygen levels, oxygen-rich rutheniummetal films, including RuO₂, are deposited.

It is also well known in the art that a surface reaction rate-limiteddeposition is required for a process to conformally coat high aspectstructures. As defined herein, “surface reaction rate-limiteddepositions” result when, for a particular temperature and co-reactantgas, the precursor gas is transported to the surface faster than it canreact on the surface. As such, the rate of film deposition is controlledby the reaction kinetics of the surface and is independent of masstransport of the precursor to the surface. Surface reaction rate-limiteddeposition is distinguished from mass transport rate-limited depositionin that, for the same temperature and co-reactant gas, the precursorreacts as fast as it transported to the surface in a mass transportlimited deposition. Under surface reaction rate-limited depositionconditions, the surface is saturated with unreacted precursor moleculesso that the deposition rate is uniform on all surfaces. At a particulartemperature, the mass transport rate of the precursor molecule can beincreased to achieve surface reaction rate-limited deposition.Alternatively, at a particular mass transport rate to a surface, thetemperature can be decreased to achieve a surface reaction rate-limiteddeposition. As stated hereinabove, this deposition regime is subject tothe incorporation of deleterious impurities in the ruthenium film.

It has been surprisingly discovered that by controlling the depositionrates at various points in the process, the ruthenium thin filmsdeposited by CVD according to the method herein have superiorcharacteristics, including full coverage, high conformality, superioradherence and high conductivity.

When forming a ruthenium film by CVD, a substrate is typically loadedinto the CVD chamber and a ruthenium precursor gas and a co-reactant gasare supplied to the chamber. A solid precursor may be sublimed togenerate the precursor gas. Alternatively, a high efficiency solidsource ampoule, such as the vaporizer delivery system described in U.S.patent application Ser. No. 10/201,518 in the name of John Gregg et al.,now issued on Jul. 26, 2005 as U.S. Pat. No. 6,921,062, incorporatedherein by reference, can be used to provide higher precursor flows atlower temperatures (for longer precursor lifetime). A liquid precursormay be directly vaporized to generate the precursor gas. Alternatively,the solid or liquid precursor may be dissolved in a solvent including,but not limited to, alkanes, alkanols and tetrahydrofuran (THF).Alkanols contemplated include ROH, where R can be straight-chained orbranched C₁-C₅ alkyl groups (e.g., Me, Et, i-Pr, n-Pr, t-Bu, n-Bu, n-Am,i-Am, t-Am, etc.). Especially useful are solvents that can also act asreducing agents in the MOCVD process. In practice, the solutioncontaining the precursor may be vaporized to generate the precursor gas.

The delivery of the precursor to the CVD chamber can be by soliddelivery or liquid delivery. As defined herein, “solid deliveryapproach” includes the heating of the solid precursor source to vaporizethe solid precursor. As the vaporized precursor is released from thesolid source, a quantity of the precursor vapor is mixed with thecarrier gas for transport to the CVD chamber. In a preferred embodiment,a solvent may be vaporized simultaneously for mixture with the carriergas. The carrier gas mixture comprising the vaporized precursor and thevaporized solvent is subsequently transported to the CVD chamber.Solvents contemplated include, but are not limited to, alkanes, alkanolsand tetrahydrofuran (THF). Alkanols contemplated include ROH, where Rcan be straight-chained or branched C₁-C₅ alkyl groups (e.g., Me, Et,i-Pr, n-Pr, t-Bu, n-Bu, n-Am, i-Am, t-Am, etc.). As defined herein,“liquid delivery approach” includes the transport of a precursor liquid(either a molten precursor or a precursor solution) to a vaporizationzone for vaporization of the precursor therein. Thereafter, thevaporized precursor is transported to the CVD chamber.

Examples of suitable ruthenium sources include ruthenocenes having theformula (Cp′)Ru(Cp″), where Cp′ and Cp″ can be the same or different andhave the general formula:

where R¹-R⁵ can be independently selected from the group consisting ofH, F, and straight-chained or branched C₁-C₅ alkyl groups (e.g., Me, Et,i-Pr, n-Pr, t-Bu, n-Bu, sec-Bu, n-amyl, i-amyl, t-amyl, etc.). Examplesof ruthenocenes contemplated herein include, but are not limited to,Ru(EtCp)₂ and Ru(Cp)₂.

Alternatively, the ruthenium source may comprise a rutheniumβ-diketonate having the formula Ru(β-diketonate)₃, where β-diketonate isdescribed by the general formula:

where R¹ and R² can be independently selected from the group consistingof H, F, straight-chained or branched C₁-C₅ alkyl groups (e.g., Me, Et,i-Pr, n-Pr, t-Bu, n-Bu, sec-Bu, n-amyl, t-amyl, etc.), andfluorine-substituted straight-chained or branched C₁-C₅ alkyl groups(e.g., Me, Et, i-Pr, n-Pr, t-Bu, n-Bu, sec-Bu, n-amyl, i-amyl, t-amyl,etc.). Specific examples of β-diketonates include, but are not limitedto:

-   -   acac=2,4-pentanedionate    -   tfac=1,1,1-trifluoro-2,4-pentanedionate;    -   thd=2,2,6,6-tetramethyl-3,5-heptanedionate;    -   hfac=1,1,1,5,5,5-hexafluoro-2,4-pentanedionate;    -   tod=2,2,7-tetramethyl-3,5-octanedionato;    -   fod=6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato;        and    -   od=2,4-octanedionato.        Preferably, R¹ and R² are independently selected from the group        consisting of C₁-C₅ fluoroalkyl groups when non-selectively        depositing a seed layer on a substrate having both dielectric        and metallic regions.

As a further alternative, the ruthenium source gas may comprise aruthenium β-diketonate neutral Lewis base adduct, having the formulaRu(β-diketonate)_(x)L_(3-x), where x can be 1 or 2, β-diketonate isdefined above, and L can be alkenes, alkynes, cycloalkenes orcycloalkynes, for example, cyclooctadiene (COD),1,5-dimethyl-1,5-cyclooctadiene (DMCOD) and 2-butyne.

In another alternative, the ruthenium source gas may comprise rutheniumcarbonyls, such as Ru(CO)₅, Ru₃(CO)₁₂, and Ru₂(CO)₉, ruthenium oxides,such as RuO₄, or ruthenium halides, such as RuCl₃. Preferably, RuCl₃ orruthenium carbonyls are used for non-selectively depositing a seed layeron a substrate having both dielectric and metallic regions.

Examples of co-reactant gases include oxygen, hydrogen, steam, nitrousoxide, ozone, carbon monoxide and carbon dioxide. In addition, solventsfor the precursor, including alkanols and tetrahydrofuran (THF), may actas liquid co-reactants. Alkanols contemplated include ROH, where R canbe straight-chained or branched C₁-C₄ alkyl groups (e.g., Me, Et, i-Pr,n-Pr, t-Bu, i-Bu, n-Bu, t-Am, i-Am, n-Am, etc.). During deposition, aninert gas such as argon or nitrogen may be used as a carrier gas forsmoothly supplying the ruthenium source gas or reactant gas and as apurge gas for purging the deposition chamber.

Examples of substrates include silicon, silicon dioxide, siliconnitride, low k materials, Co(WP) capping layers, copper, hafniumsilicate, hafnium oxide, hafnium silicate nitride (Hf—Si—O—N), hafniumoxide nitride (Hf—O—N), titanium nitride, titanium aluminum nitride(TiAlN), tantalum nitride, tantalum pentoxide (Ta₂O₅), barium strontiumtitanate (BST) or lead zirconate titanate (PZT).

In conventional CVD methods, a ruthenium film is typically depositedunder constant process conditions, including temperature, pressure,oxygen content and ruthenium precursor content. In accordance with theembodiments of the present invention, however, ruthenium films areformed by changing the process conditions during the CVD procedure.

In one aspect, the present invention relates to a multi-step method fordepositing ruthenium thin films having high conductivity and superioradherence to the substrate. The method includes the deposition of aruthenium nucleation layer followed by the deposition of a highlyconductive ruthenium upper layer. Together, the ruthenium nucleationlayer and the ruthenium upper layer make up the “ruthenium thin film” ofthe present invention.

The first deposition step of the CVD method is carried out underoxidizing conditions at temperatures ranging from about 250° C. to about340° C. If the temperature is less than about 250° C., the depositionrate is too slow for full coverage of the substrate. In contrast, if thetemperature is greater than about 340° C., the nuclei formed on thesubstrate grow separately and do not coalesce until the films are quitethick, e.g., greater than 500 Å. By maintaining the CVD temperaturesbetween about 250° C. and 340° C., a continuous film of less than 50 Å,and more preferably less than 35 Å can be deposited. In a particularlypreferred embodiment, the nucleation layer deposited during the firststep has a thickness of about 5 Å to about 10 Å.

The pressure of the CVD chamber during the first deposition step is fromabout 5 Torr to about 0.1 Torr, preferably from about 2 Torr to about0.2 Torr. In a particularly preferred embodiment, the pressure of theCVD chamber during the first deposition step is from about 1 Torr toabout 0.4 Torr.

In addition to temperature and pressure, the oxygen content during thefirst deposition step of the deposition process is important. Whendepositing the ruthenium nucleation layer, the precursor:co-reactant gasmixture should have an oxygen content greater than about 30 mole %oxygen, based on the total number of moles in the process gas mixture.More preferably, the oxygen content should be from about 80 mole % toabout 95 mole %, based on the total number of moles in the process gasmixture.

The rate of introduction of the ruthenium precursor gas during the firstdeposition step of the CVD method should be from about 2 μmol/min toabout 100 μmol/min, preferably about 15 μmol/min to about 30 μmol/min,said rate of introduction of ruthenium precursor being adjusted tomaintain surface reaction rate-limited deposition. In a particularlypreferred embodiment, the rate of introduction of the rutheniumprecursor gas during the first deposition step is about 15 μmol/min whenthe wafer temperature is 280° C. In another particularly preferredembodiment, the rate of introduction of the ruthenium precursor gasduring the first deposition step is about 30 μmol/min when the wafertemperature is 300° C.

The deposition rate of the ruthenium nucleation layer deposited duringthe first deposition step of the process should be from about 20 Å/minto about 1 Å/min, more preferably from about 10 Å/min to about 1 Å/min,most preferably about 5 Å/min to about 1 Å/min.

In a preferred embodiment, the ruthenium nucleation layer is depositedusing a precursor chosen for its ability to substantially cover thesubstrate surface with the thinnest possible layer. “Substantially”covering is defined herein as covering at least 90% of the exposedsubstrate surface. For example, thin films deposited using rutheniumβ-diketonates such as Ru(thd)₃ as the precursor provide more substantialcoverage and lower resistivity than thin films deposited using aRu(EtCp)₂ precursor. As described herein, the ruthenium nucleation layerwill be continuous and adherent to the underlying substrate materials.It is further noted that the nucleation layer will have good electricalcontact, i.e., low contact resistance, with metallic regions of theunderlying substrate materials.

The second deposition step of the CVD method is carried out underconditions that produce films with a low percentage of impurities suchas oxygen and/or carbon. Preferred films consist essentially of thedeposited metal film having an impurity content of less than about 2atomic %.

During the second deposition step, a ruthenium upper layer is chemicallyvapor deposited onto the ruthenium nucleation layer deposited during thefirst deposition step. Because the deposited ruthenium nucleation layeris continuous and has superior adherence to the underlying substrate andthe ruthenium upper layer is continuous and has superior adherence tothe underlying nucleation layer, the ruthenium thin film of theinvention has superior adhesion to the underlying substrate.

To ensure the deposition of a continuous upper layer, the seconddeposition step process conditions should be more reducing than thefirst deposition step process conditions. This can be achieved bylowering the oxygen content of the process gas stream, lowering theabsolute pressure of the process, or increasing the temperature of thesubstrate.

The second deposition step of the CVD method is carried out attemperatures equivalent to, or higher than, the temperatures of thefirst deposition step of the CVD method. Preferably, the temperature ofthe second deposition step is from about 250° C. to about 400° C., morepreferably from about 300° C. to about 400° C., most preferably fromabout 350° C. to about 400° C. Notably, in applications wheretemperatures above 400° C. are compatible with the integration,temperatures above 400° C. may be advantageous.

The pressure of the CVD chamber during the second deposition step iscarried out at pressures equivalent to, or less than, the pressures ofthe first step of the CVD method. Preferably, the pressure of thechamber during the second deposition step is from about 2 Torr to about0.1 Torr, more preferably from about 1 Torr to about 0.2 Torr. In aparticularly preferred embodiment, the pressure of the CVD chamberduring the second deposition step is less than about 0.4 Torr.

With regards to process gases, the co-reactant gas used during thesecond deposition step is preferably an oxygen gas:hydrogen gas mixturehaving a molar percentage ratio between about 1:1 and about 1:3,preferably about 1:2. Preferably, the precursor:co-reactant gas mixturehas an oxygen content less than about 50 mole % oxygen, based on thetotal number of moles in the process gas mixture. More preferably, theoxygen content is from about 20 mole % to about 1 mole %, based on thetotal number of moles in the process gas mixture.

The rate of introduction of the ruthenium precursor gas during thesecond deposition step of the CVD method should be less than that whichkeeps the process in the surface reaction rate-limited depositionregime. Preferably, the precursor flow rate of the second depositionstep is chosen to produce the lowest resistivity films in the leastamount of time within the constraints of the chosen integration scheme.It is preferred that the rate of introduction of ruthenium precursorduring the second deposition step is the same as, or less than, the rateof introduction of ruthenium precursor during the first deposition step,most preferably between about 5 μmol/min and 20 μmol/min.

The deposition rate of the ruthenium upper layer deposited during thesecond deposition step of the process should be from about from about 15Å/min to about 1 Å/min, more preferably from about 10 Å/min to about 1Å/min, most preferably about 5 Å/min to about 1 Å/min.

In a particularly preferred embodiment, the final ruthenium thin filmdeposited during the multi-step CVD process is a continuous film havinga high density and superior adherence to the underlying substrate.Unexpectedly, if the ruthenium thin film is deposited onto the substrateusing lower deposition rates, as described herein, the ruthenium thinfilm will have a low resistivity relative to its thickness. For example,using the CVD method taught herein, films have been deposited with aresistivity of less than 100 μΩ-cm at a thickness of about 25 Å, aresistivity of 20 μΩ-cm at a thickness of about 85 Å, and a resistivityof less than 10 μΩ-cm at a thickness of about 350 Å.

A further embodiment of the CVD method includes an intermediateannealing step, wherein the ruthenium nucleation layer is annealed in areducing atmosphere, e.g., hydrogen gas, prior to deposition of theruthenium upper layer thereon. The ruthenium nucleation layer isannealed in a H₂, H₂/N₂ or H₂/Ar environment at about 200° C. to about400° C., most preferably the temperature of the second deposition step.Annealing the nucleation layer reduces both the oxygen and carboncontent of the ruthenium nucleation layer, concomitantly improving itselectrical conductivity, without decreasing its adhesion to a substratesurface. In addition, post-deposition anneals are contemplated forfurther improving conductivity of the ruthenium thin film depositedduring the two-step CVD process.

In a further embodiment, the CVD method may be a pulsed process, whereinthe first deposition step is carried out as described hereinabove,followed by a deoxygenating step, wherein the nucleation layer isprocessed in the absence of ruthenium precursor. Preferably, thedeoxygenating step in this embodiment is carried out in the absence ofprecursor under process conditions that are more reducing than those ofthe first deposition step process, including lower pressure, loweroxygen content, higher temperature and higher reducing agent, e.g.,hydrogen, content. This two step process may be successively andcontinuously repeated until the ruthenium thin film of desired thicknessis deposited. In a particularly preferred embodiment, the temporallength of deposition of steps 3, 5, 7, etc., are approximately equal andless than the length of deposition of step 1. Notably, the rate ofprecursor mass transport is preferably maintained at a high enough levelduring the deposition steps, i.e., steps 1, 3, 5, etc., to maintain asurface reaction rate-limited deposition. This ensures that highlyconformal films can be deposited with low impurity content and lowresistivity on high aspect ratio substrates.

In a still further embodiment, the ruthenium precursor employed duringthe deposition of the nucleation layer is different than the rutheniumprecursor employed during the deposition of the upper layer of the film.For example, ruthenium β-diketonates are preferred for the deposition ofthe nucleation layer because of their full coverage of the substrate,low resistivity, and good adhesion. During the deposition of the upperlayer of the film, ruthenocenes are preferred because of their highergrowth rate, low resistivity and superior adhesion to the underlyingruthenium nucleation layer. The particular precursors used for thedeposition of each layer can be chosen by experimentation as isgenerally practiced in the art. Preferably, the deposition of thenucleation layer using a first ruthenium precursor occurs at a lowerdeposition rate than the deposition of the upper layer using a secondruthenium precursor.

The CVD process described herein may be modified for capping copperconductors in a damascene structure with a ruthenium thin film. Forexample, ruthenium films may be deposited from a ruthenium precursorgas, e.g., Ru(EtCp)₂, under inert or reducing atmospheres at hightemperatures, e.g., 400° C. Under these process conditions, thepercentage of oxide impurities in the deposited ruthenium thin film isnegligible. In a preferred embodiment, hydrogen is used as a co-reactantgas, which enhances the surface selectivity of the deposition process sothat ruthenium capping layers are deposited only on the exposed copperof the substrate.

The features and advantages of the invention are more fully shown by theillustrative examples discussed below.

Example 1 Comparison of Nucleation Layers Deposited using Ru(EtCp)₂Precursor Relative to those Deposited using Ru(thd)₃ Precursor

An experiment was designed to compare Ru(EtCp)₂ to Ru(thd)₃ precursorsover a wide range of process conditions to investigate the precursorsrelative utility when depositing very thin conductive films. The molarflow rate of the precursor was held constant at 15 μmol/min, thedeposition time was held constant at 600 seconds and the total gas flowwas held constant at 1000 sccm. Pressure, temperature, and reactive gasmixture were varied as shown in Table 1. The resulting film thicknesses(measured by XRF calibrated to RBS) and film resistivities (calculatedusing the physical thickness measured by spectroscopic ellipsometry) ofthe deposited films are also provided in Table 1. If film resistivity isnot provided, it was too high to measure, indicating that the film wasnot continuous and/or contained substantial quantities of impurities. Ofnote, contrary to the teaching of Won et al. (see U.S. Pat. No.6,680,251), which teaches seed layer deposition in the 5-50 Torrpressure regime using a wide range of precursors, including rutheniumβ-diketonates, little or no film was deposited using Ru(thd)₃ under thehigher pressure conditions.

TABLE 1 Process conditions comparison for the deposition of Ru(EtCp)₂precursors relative to Ru(thd)₃ precursors. co- reactant Ru(EtCp)₂Ru(thd)₃ p/Torr T/° C. H₂ O₂ Å (XRF) μΩ-cm Å (XRF) μΩ-cm 0.8 225 4% 2% 30 0.8 300 4% 0% 5 0 0.8 300 0% 80% 42 1400 32 212 0.8 400 4% 2% 27 262.5 225 4% 0% 0 0 2.5 225 0% 80% 2 0 2.5 300 4% 2% 1 2 2.5 400 4% 0% 1 02.5 400 0% 80% 97 25,000 5 8.0 225 4% 2% 3 0 8.0 300 4% 0% 4 0 8.0 3000% 80% 87 490 2 8.0 400 4% 2% 86 4

Further experiments were performed for each precursor using the mostpromising process conditions from the first set of experiments reportedin Table 1. For Ru(EtCp)₂, a range from 2-80% O₂ and from 260-345° C.was further examined at 8.0 Torr. It was determined that the lowestresistivity film (350 μΩ-cm in a film that was 91 Å by XRF and 317 Å byellipsometry) was deposited at 80% O₂ and 345° C. For Ru(thd)₃, a rangefrom 0.4-1.6 Torr and 260-345° C. was further examined at 90% O₂. It wasdetermined that the lowest resistivity film (138 μΩ-cm in a film thatwas 23 Å by XRF and 59 Å by ellipsometry) was deposited at 1.0 Torr and300° C. The lower resistivity in thinner films deposited using theRu(thd)₃ precursor indicates that Ru(thd)₃ is a superior precursor forvery thin film deposition.

Example 2 Process Conditions for the Deposition of the Nucleation Layer

The first deposition step of the CVD method taught herein results in thedeposition of a ruthenium nucleation layer. Table 2 illustrates fourdifferent ruthenium nucleation layers deposited according to the methodtaught herein and the conditions and characteristics of each layer:

TABLE 2 Process conditions for deposition of ruthenium nucleation layerProcess Reactant resistivity/ number T/° C. p/Torr gases (wt %)Deposition rate Density Phase μΩ-cm 1 400 0.8  2% O₂/4% H₂ 2.7 Å/min 41%metal ∞ 2 300 0.8 90% O₂ 3.0 Å/min 29% oxide 214 3 300 1.0 90% O₂ 3.0Å/min 50% metal 138 4 300 0.8  2% O₂/4% H₂ 2.5 Å/min 29% metal ∞

In all cases, the ruthenium source gas was a vaporized solution ofRu(thd)₃. It was initially thought that process numbers 1 and 4, usingco-reactant gases having lower oxygen content, would result in adeposited ruthenium nucleation layer with less residual oxygen therebyproviding a more conductive layer. However, the resistivity was too highto measure, indicating that the films were not continuous. Without beingbound by theory, the ruthenium nucleation layers deposited in processnumbers 1 and 4 may have been “island-like” with nuclei that failed tocoalesce because the ruthenium layer deposited was too thin. Processnumbers 2 and 3 both produced continuous films.

Example 3 Process Conditions for Depositing the Upper Layer

The second deposition step of the CVD method results in the depositionof a ruthenium upper layer yielding the final ruthenium thin film, astaught herein in one aspect of the invention. Table 3 illustrates threedifferent experiments and the results of each, wherein the rutheniumnucleation layer and upper layer were deposited according to the firstand second set of process conditions, respectively:

TABLE 3 Process conditions for deposition of ruthenium thin filmsProcess T₁/° C. p₁/Torr gas₁ (wt %) prec. rate₁ resistivity/ Adhesionnumber T₂/° C. p₂/Torr gas₂ (wt %) prec. rate₂ Density μΩ-cm tape 5 3001.0 90% O₂ 15 μmol/min 44% 172 fail 300 0.4 30% O₂ 15 μmol/min 6 300 0.890% O₂ 15 μmol/min 80% 14.5 pass 400 0.4  2% O₂/4% H₂ 15 μmol/min 7 3000.8 90% O₂ 15 μmol/min 78% 11.2 pass 400 0.4  2% O₂/4% H₂ 7.5 μmol/min

Process 5 represented a ruthenium thin film having a nucleation layergrown according to conditions enumerated in process number 3hereinabove. Although the resistivity of the ruthenium thin filmdeposited during the two-step CVD process was measurable, indicating acontinuous film, the ruthenium thin film had poor adhesion to theunderlying substrate as indicated by the tape adhesion test. In otherwords, the film deposited according to the Process 5 conditions is not apeel-resistant layer film.

Processes 6 and 7 represented a ruthenium thin film having a nucleationlayer grown according to the conditions enumerated in process number 2hereinabove. The ruthenium thin film deposited during the two-step CVDprocess in processes 6 and 7 displayed low resistivities and superioradhesion to the underlying substrate, i.e., peel-resistant layer filmswere deposited. Resistivity of the ruthenium thin film was improved byadjusting the rate of ruthenium precursor delivery during step two ofthe process, e.g., process number 7.

FIG. 1 represents a schematic of the process conditions associated withdeposition process 7 as a function of time (arbitrary units), where thesharp change in process conditions represents the change from step oneprocess conditions, i.e., nucleation layer deposition, to the step twoprocess conditions, i.e., upper layer deposition.

FIG. 2 illustrates the resistivity of ruthenium thin films as a functionof film thickness for ruthenium thin films deposited according to theconditions enumerated in process 7 hereinabove.

Example 4 Process Conditions for Surface Reaction Rate-LimitedDeposition

In order to deposit conformal films over high aspect ratio structures,surface reaction rate-limited depositions are preferred. To determinethe precursor flow rates necessary to maintain surface reactionrate-limited deposition, experiments at two temperatures, 280° C. and300+ C., were performed. Table 4 summarizes the film thicknesses thatwere deposited at the various temperatures, precursor rates anddeposition times.

TABLE 4 Film thicknesses and resistivities relative to precursor flowrates Process co-reactant Precursor Thickness/ resistivity/ number T/°C. p/Torr (%) rate Time/sec Ellips.Å μΩ-cm 8 280 1.0 90% O₂ 15 μmol/min300 49 234 9 280 1.0 90% O₂ 30 μmol/min 300 51 238 10 300 1.0 90% O₂ 15μmol/min 300 94 203 11 300 1.0 90% O₂ 30 μmol/min 300 125 180 12 300 1.090% O₂ 30 μmol/min 90 27 1300 13 300 1.0 90% O₂ 60 μmol/min 90 28 1050

Comparing process 8 to process 9, it can be seen that at 280° C., filmthickness is independent of precursor rate over the range from 15-30μmol/min. In contrast, comparing process 10 to process 11, both at 300°C., the film thickness increases significantly as precursor rateincreases from 15 μmol/min to 30 μmol/min. Comparing process 12 toprocess 13, both at 300 ° C., the film thickness is independent ofprecursor rate over the range from 30-60 μmol/min. It was determinedthat the precursor rate of 15 μmol/min was sufficient to maintainsurface reaction rate-limited deposition at 280° C., and 30 μmol/min wassufficient to maintain surface reaction rate-limited deposition at 300°C.

Example 5 Pulsed Process for Upper Layer of Ruthenium Film

Based on the process conditions determined in Examples 1 through 4, twopulsed processes were performed, one at 280° C. and the other at 300° C.The schematic of process variables as a function of time (arbitraryunits) is shown in FIG. 3. In each case, the length of the firstdeposition step was determined by taking the shortest time at whichresistivity could be measured, however, other methods of determiningfull coverage could also be used. Following nucleation layer depositionat 1 Torr and 90% O₂, the deoxygenating step (devoid of precursor) wascarried out at 9 Torr and 4% H₂. Temperature remained constantthroughout the deposition of the nucleation layer and the subsequentdeoxygenating of the ruthenium layer. Film thickness was varied bychanging the number of subsequent deposition cycles. FIG. 4 illustratesthe resistivity of the films deposited by pulsing as a function of filmthickness, labeled process B ((▴) pulsed process at 280° C., (▪) pulsedprocess at 300° C.), relative to films deposited at continuous firstdeposition step conditions, labeled process A ((♦) continuous firstdeposition step at 280° C., (●) continuous first deposition step at 300°C.). Continuous first deposition step conditions (process A) include apressure of 1 Torr and 90% O₂, which result in a surface reactionrate-limited deposition. The resulting films are conformal, but a higherpercentage of impurities are incorporated into the films of continuousprocess A compared to pulsed process B.

Accordingly, while the invention has been described herein in referenceto specific aspects, features and illustrative embodiments of theinvention, it will be appreciated that the utility of the invention isnot thus limited, but rather extends to and encompasses numerous otheraspects, features and embodiments. Accordingly, the claims hereafter setforth are intended to be correspondingly broadly construed, as includingall such aspects, features and embodiments, within their spirit andscope.

1. A method for depositing a ruthenium thin film onto a substrate, saidmethod comprising: (a) depositing a nucleation layer comprisingruthenium onto the substrate by chemical vapor deposition, wherein thenucleation layer is deposited using a nucleation layer mixturecontaining a first ruthenium precursor under nucleation layer CVDconditions; and (b) depositing an upper layer comprising ruthenium ontothe nucleation layer by chemical vapor deposition, wherein the upperlayer is deposited using an upper layer mixture containing a secondruthenium precursor, different from the first, under upper layer CVDconditions.
 2. The method of claim 1, wherein the first and secondruthenium precursors are selected from the group consisting ofruthenocenes, ruthenium β-diketonates , fluorinated rutheniumβ-diketonates, ruthenium carbonyls, ruthenium oxides and rutheniumhalides.
 3. The method of claim 1, wherein the first ruthenium precursoris selected from the group consisting of ruthenium β-diketonates,fluorinated ruthenium β-diketonates, ruthenium carbonyls, and rutheniumoxides.
 4. The method of claim 1, wherein the first ruthenium precursoris a ruthenium β-diketonate and the second ruthenium precursor is aruthenocene.
 5. The method of claim 4, wherein the ruthenocene has theformula (Cp′)Ru(Cp″), where Cp′ and Cp″ can be same or different andhave the general formula:

where R¹-R⁵ are independently selected from the group consisting of H,F, and straight-chained or branched C₁-C₅ alkyl groups.
 6. The method ofclaim 4, wherein the ruthenocene comprises Ru(EtCp)₂ or Ru(Cp)₂.
 7. Themethod of claim 4, wherein the ruthenium β-diketonate has the formulaRu(β-diketonate)₃, where β-diketonate has the general formula:

where R¹ and R² are independently selected from the group consisting ofH, F, straight-chained or branched C₁-C₅ alkyl groups, andfluorine-substituted straight-chained or branched C₁-C₅ alkyl groups. 8.The method of claim 4, wherein the ruthenium β-diketonate comprises acompound selected from the group consisting of: tris(2,4-pentanedionate)ruthenium (Ru(acac)₃); tris(1,1,1-trifluoro-2,4-pentanedionate)ruthenium (Ru(tfac)₃); tris(2,2,6,6-tetramethyl-3,5-heptanedionate)ruthenium (Ru(thd)₃); tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)ruthenium (Ru(hfac)₃); tris(2,2,7-tetramethyl-3,5-octanedionato)ruthenium (Ru(tod)₃); tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) ruthenium(Ru(fod)₃); and tris(2,4-octanedionato) ruthenium (Ru(od)₃).
 9. Themethod of claim 4, wherein the ruthenium β-diketonate comprises Ru(thd)₃or Ru(hfac)₃.
 10. The method of claim 1 wherein the deposited nucleationlayer comprises ruthenium metal.
 11. The method of claim 1, wherein thenucleation layer CVD conditions include a temperature of from about 250°C. to about 340° C. and a flow rate of the first ruthenium precursor offrom about 15 μmol/min to about 30 μmol/min and the co-reactant gas isoxygen and is present in a concentration of from about 80 mole % toabout 95 mole % and the upper layer CVD conditions include a temperatureof from about 250° C. to and about 400° C. and a flow rate of the secondruthenium precursor of from about 5 μmol/min to about 20 μmol/min andfrom about 1 mole % to about 10 mole % of the second rutheniumprecursor/co-reactant gas mixture comprises oxygen.
 12. The method ofthe claim 11, wherein second co-reactant gas comprises anoxygen:hydrogen gas mixture.
 13. The method of claim 12, wherein themole ratio of oxygen to hydrogen is from about 1:1 to about 1:3.
 14. Themethod of claim 1 wherein the flow of the first ruthenium precursor issufficient to maintain a surface reaction rate-limited deposition andthe flow of the second ruthenium precursor is less than that needed tomaintain a surface reaction rate-limited deposition.
 15. The method ofclaim 14, wherein the nucleation layer CVD conditions include atemperature of from about 250° C. to about 340° C. and the firstco-reactant gas comprises more than 30 mole % of the first rutheniumprecursor/co-reactant gas mixture.
 16. The method of claim 1, whereinthe upper layer CVD conditions are more reducing than the nucleationlayer CVD conditions.